The present disclosure relates generally to the field of Radio-Frequency (RF) transistors. In particular, a system employed to provide increased stability in the testing and measurement of RF transistors are described.
Design requirements to keep RF transistors and other amplifiers and three-terminal discrete devices stable are usually in conflict with the needs of a Source Management Unit (SMU) when conducting DC measurements on these devices. In particular, DC testing of such RF devices tends to cause the RF device to break into oscillation. As a result, many RF devices simply could not be DC tested. Thus, there exists a need for a method of testing RF transistors that improve upon and advance the design of known methodologies for testing these components. Examples of new and useful systems relevant to the needs existing in the field are discussed below.
In this regard, SMU's are often used to test high-speed devices (speeds greater than 1 Mhz) such as, transistors and integrated circuit amplifiers. DC I/V (current/voltage) curves of transistors and IDDQ measurements of RF amplifiers are common tests conducted on these devices. The symbol IDDQ has two meanings. IDDQ is commonly used to refer to the quiescent supply current and may also be used to refer to a test methodology that is based on taking quiescent supply current (IDDQ) measurements. Thus, IDDQ as testing methodology is one based on measuring the quiescent supply current of a device-under-test (DUT).
Each of these devices has one thing in common, gain, which mandates that some special care be taken when using or testing these devices. As is well-known in the art, any device with gain has the potential to oscillate if the output is allowed to couple back to the input with zero phase while the amplifier gain is greater than one. When these high-speed amplifiers are used in their intended application, care must be taken so that the output does not couple back to the input with a phase-aligning delay. Further, in the case of very high-speed amplifiers, additional care must be taken to ensure that the input and output lines of these devices are properly terminated to eliminate reflections. Reflections from the amplifier output can couple to the amplifier input, causing the amplifier to oscillate. In this case, a reflection could couple energy from the output of the amplifier to the input of the amplifier, generating a zero phase condition, as previously described above.
Previous high-speed devices such as transistors and amplifiers were typically connected to SMU's with long banana or triaxial cables. In each case, the long cables (transmission lines) were not properly terminated nor did they have the correct RF impedance to eliminate unwanted oscillations. As a result, many high-speed devices would oscillate when basic I/V measurements were attempted in the manner described above.
These triaxial cables, often referred to as a triax cables for short, are a type of electrical cable similar to a coaxial cable (coax for short), but with the addition of an extra layer of insulation and a second conducting sheath. Thus, the triax cables provide greater bandwidth and rejection of interference than the coax cable. Ideally, triax cables exhibit an impedance of about 100 ohms from the inner conductor to the outer shell.
Previously known methods and systems to abate such unwanted isolations called for the inner shielding of the triax cables to provide a “guard” for the Hi and Sense Hi input connections to the SMU. The guard frequency is rolled-off far below the SMU loop closure to prevent the SMU from oscillating due to the unwanted condition described above, referred to as a “guard-ring oscillator.” The split guard above is accomplished by driving a cable guard with a resistor. The resistive guard will roll-off with a frequency allowing the guard to “float” at high frequencies. As a result, this inner shielding, or “guard conductor” in the triax cables will assume an appropriate RF voltage in accordance with its position between the inner and outer shielding of the triax cables for all frequencies well above the guard roll-off frequency.
Accordingly, improvements directed towards testing high-speed RF devices that reduce or eliminate unwanted oscillations are desirable.